As the nuclear power industry matures, the issue of safe storage, transport and disposition of spent nuclear fuel becomes increasingly critical. While the original intent in the industry was to reprocess spent fuel, that option has been delayed by a variety of cost and political concerns. Central storage of spent nuclear fuel in containers pending resolution of those issues, and in anticipation of future reprocessing, has been an industry goal, and storage containers for spent fuel have been designed in many cases in anticipation of hauling quantities of spent fuel over long distances to central storage facilities. The hauling of radioactively and thermally hot nuclear materials over public thoroughfares is highly regulated, and the containers for such materials must meet rigid safety and engineering standards. For this reason, the design and construction of containers for these materials is complex and expensive. However central storage of spent nuclear fuel has also been met with difficult regulatory and political issues, and to date has not been implemented on a large scale. Instead, nuclear power plants have resorted to on-site storage of spent fuel. Several plants have constructed expensive storage facilities for spent fuel utilizing water pools in which containers of spent rods are immersed. The surrounding water provides thermal cooling of the thermally "hot" materials, and affords radiation protection and isolation from the environment. However, pool capacity is limited and in some cases exhausted, and nuclear power plants are pursuing new and more economic strategies for handling spent fuel, including on-site dry storage.
Previously, thin walled containers were developed and utilized for ease in handling. These containers could be lowered directly into the fuel storage pool, loaded with spent fuel rods, and once sealed, removed from the fuel pool and transferred to an outdoor storage area. This technology was attractive since it could be handled easily by utilities and due to its structural advantages, transported directly from the utilities to a central storage facility or to an underground repository.
While thin walled metal transport and storage casks proved technically feasible, they also proved expensive. New technology, primarily storage only concrete containers were developed at a cost 6-8 times less expensive when compared to the metal containers. These concrete containers were approved for storage only and involved complex handling equipment and procedures since the large concrete containers could not be lowered into the fuel storage pool. In order to effect transfer, a thin walled metal transfer cask was lowered into the fuel pool, loaded with fuel and sealed. Its contents were transferred outside the fuel pool.
Engineering issues for on-site dry storage containers are primarily thermal cooling and radiation shielding. Radiation shielding can effectively be incorporated in storage containers using thick steel walls or thick walls of other high radiation cross section materials, but this approach to radiation shielding results in a very heavy and very expensive container. The preferred solution from a cost standpoint is to use a concrete container. Concrete typically contains a significant amount of hydrated water which acts as an effective neutron radiation shield, and also contains iron aggregates which are effective in shielding gamma radiation. However, with anticipated storage times of months and years, or even decades, thermal issues are paramount. Storage of radioactive materials involves the use of sealed containers, and it is difficult to effectively cool materials that are generating heat inside of a sealed container. It is especially difficult if the container is concrete, or contains a concrete layer, as concrete is a poor heat transfer medium. Thus a satisfactory concrete storage container for thermally hot nuclear materials has not been available in the industry.